Prototype Portal + Ugin's Nexus + Phantom Train

Prototype Portal

Ugin's Nexus

Phantom Train

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. Activate Phantom Train by acrificing the Ugin's Nexus token.
3. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
4. Resolve the Phantom Train ability
5. Repeat each turn.